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I tried a setup similar to yours before but in place of an air stone I used an aquarium power head to blow the bubbles at a constant rate and allow them to further diffuse in the water. It definitely worked and Aldrovanda grew well before the power head broke.

I use 10 and 5L CO2 reactors for my utrics and aldros, I also added a 1 way valve to stop reverse flow as the reactors are at a lower level than some of the smaller ponds they are used in, my 5L last about a month to two before they need to be changed, I use air stones in the smaller ponds but if you can bury the hose under 3-4cm of sand the bubbles come out relatively evenly over a 6-7cm radius and are more constant.
Alternatively/additionally you can add rhizome plants, I use typha and sarra's, they add a fair bit of CO2.

I have been using them for a few years now and they do increase growth enough to be noticed, but can also increase algea if you are using high light like I often do, horn snail and daphnia help counter this, as do other companion plants like nymphoides and marsilea.